Effects of nutrients and parasitism on the density of a salt marsh planthopper suppressed by within‐trophic‐level interactions

Abstract

Abstract. 1. Competition, food quality, and pressure from natural enemies have all been established as important determinants of phytophagous insect densities. Examining how these forces interact, however, is important for furthering our understanding of insect population and community ecology.2. In this paper the results of a factorial field experiment are reported in which the effects of within‐trophic‐level interactions on the top‐down and bottom‐up forces influencing the density of a salt marsh planthopper were examined.3. Plant quality was increased by adding nitrogen fertiliser, and parasitism was decreased through the use of yellow sticky traps that removed hymenopteran parasitoids. These treatments were applied to plots with either low or high densities of a lepidopteran stem borer, which has been shown to negatively affect the planthopper.4. Addition of nitrogen increased planthopper density significantly, but only on plots with low stem borer densities. Yellow sticky traps significantly reduced parasitism of planthopper eggs, but this resulted in increased planthopper density only on plots with low stem borer density. Thus, where stem borer densities were high, the effects of fertiliser and reduced parasitism on planthopper densities were suppressed.5. The results of this study show that negative interactions between herbivores can affect vertical trophic linkages, and can greatly modify the strengths of top‐down or bottom‐up effects on herbivore density.